Support passing partials a filtered context object

@tbranyen, here's my use-case: I have a partial that is used to render an article's banner image in a standardized manner. However, depending on the page, that image's context object may not be usable without pre-filtering. I could do something in the template, but I would prefer to be able to contextually transform the passed object based on the page context: something like this, e.g.
```tmpl
{%-- homepage --%}
{%each posts as post %}
<div class="post">
  {%partial post-banner post.image %}
</div>
{%endeach%}
```
```tmpl
{%--
category archive view: category.featured_image has a different structure
from post.image, and convert-image is used to produce a valid context object
--%}
{%each categories as category %}
<div class="category">
  {%partial post-banner category.featured_image|convert-image %}
</div>
{%endeach%}
```
I'd be curious for your thoughts; if this is valid, I'd appreciate a walkthrough of the code for this grammar, so that I could take a crack at a patch.